Transmission shifting, in an automotive powertrain, is generally scheduled in relation to predefined relationships between vehicle speed and an engine load indicator, such as throttle position or manifold pressure. Representative engine throttle (TPS) vs. vehicle speed (Nv) relationships for first gear-to-second gear and second gear-to-third gear upshifts of an automotive transmission are graphically depicted in FIG. 2 by the traces 1-2 and 2-3, respectively. A 1-2 upshift is initiated, for example, when the TPS vs. Nv operating point of the vehicle crosses the 1-2 upshift line.
At throttle openings above a detent threshold (KDETENT), such as 90%, upshifting is scheduled at predefined vehicle speed thresholds (designated Nv(1-2) and Nv(2-3) in FIG. 2) regardless of the exact throttle position. These speed thresholds are generally designed to prevent the engine speed from exceeding a limit value, and such shifts are referred to as detent upshifts. In certain instances, more accurate detent upshift control may be achieved by substituting engine speed thresholds for the vehicle speed thresholds.
In either case, the underlying difficulty is that a variable amount of shift delay time typically elapses between the initiation of a shift command and the point at which the upshift actually occurs. The engine speed continues to rise in the course of the delay, and the engine speed limit may be exceeded before the shift actually occurs. This occurs primarily in high performance vehicles, and engine governor controls are generally employed to prevent engine damage.